Bluetooth is a worldwide specification for a small low-cost radio targeting various short-range wireless connectivity applications. Bluetooth networks are intended to link mobile computers, mobile phones, other portable handheld devices and provide Internet connectivity. The Bluetooth standard uses a packet switching protocol employing frequency hopping spread spectrum (FHSS) at 1600 hops/sec with a current maximum data rate of 3 Mb/s, which is referred to as the “Enhanced Data Rate” (EDR), as it was a later extension to the basic rate of 1 Mb/s from the time the Bluetooth standard was first introduced. Bluetooth radios operate worldwide in the 2.4 GHz unlicensed Industrial Scientific Medicine (ISM) band using a frequency range of 2402-2480 MHz. For carrier frequencies, the following relation holdsf=2402+k[MHz],k=0, . . . , 78  (1)
A total of 79 RF channels may be used in this frequency band. The RF bandwidth of each channel is about 1 MHz.
A frequency hopping transceiver is used to combat interference and fading. The symbol rate is 1 Ms/s, corresponding to a bit rate of 1 Mb/s, 2 Mb/s or 3 Mb/s. For full duplex transmission, a Time-Division Duplex (TDD) scheme is used wherein information is exchanged through packets in the two directions alternately. Each packet is transmitted on a different hop frequency. A packet nominally covers a single slot, but can also be extended to cover three or five slots.
The slotted channel is divided into time slots, each having a nominal slot length of 625 μs. The time slots are numbered according to the Bluetooth clock of the piconet master. The slot numbering ranges from 0 to 227 −1 and is cyclic with a cycle length of 227. In a Bluetooth piconet, which is a network of multiple synchronized devices, one device, dictating the timing, is defined as the master, and up to seven others may be acting as its slave devices, and adapt to this timing. The master transmits to a slave (or broadcasts to multiple slaves simultaneously) in even-numbered time slots only, and a slave may transmit back to the master in odd-numbered time slots only. The packet start is aligned with the slot start and timing compensation is continuously engaged in all active slaves to maintain synchronization with the time-slot timing dictated by the master.
Bluetooth transceivers use a time-division duplex (TDD) scheme to communicate, alternately transmitting and receiving in a synchronous manner. The piconet is synchronized by the system clock of the master. The Bluetooth Device Address (BD_ADDR) of the master determines the frequency hopping sequence and the channel access code. The system clock of the master determines the phase in the hopping sequence. The master controls the traffic on the channel by a polling scheme. The slaves adapt their native clocks with a timing offset in order to match the master clock.
The channel is represented by a pseudo-random hopping sequence hopping through all 79 RF channels or a modified limited hopping sequence which offers enhanced coexistence performance. The hopping sequence is unique for the piconet and is determined by the Bluetooth device address of the master; the phase in the hopping sequence is determined by the Bluetooth clock of the master. The channel is divided into time slots where each slot corresponds to an RF hop frequency. Consecutive hops correspond to different RF hop frequencies. The nominal hop rate is 1600 hops/s. All Bluetooth units participating in the piconet are time and hop synchronized to the channel.
A problem arises, however, in that the number of devices operating in the 2.4 GHz unlicensed band is increasing at an ever faster rate causing an increased likelihood of congestion. This results in increased interference caused by one wireless system to other wireless systems operating in proximity. In the case of Bluetooth piconets, it is often necessary to collocate many independent coexisting Bluetooth piconets within the same environment where frequent packet collisions between the piconets could significantly degrade their performance.
It is often the case that multiple wireless systems operate close enough to each other to potentially affect one another resulting in noticeably degraded performance for one or more of them. An example is shown in FIG. 1 where a Bluetooth system 10 operates in the vicinity of a wireless LAN (WLAN) system 20. A plurality of Bluetooth devices 12 acting as slave devices and one Bluetooth device 14 acting as a master form a piconet. The wireless LAN 20 comprises a plurality of wireless devices 24, such as 802.11b (WLAN) devices, and an access point 22.
As described above, Bluetooth is a frequency hopping system and uses fixed length packets transmitted during well-defined time slots in a TDMA fashion. Packets are transmitted in the ISM band between 2402 and 2480 MHz (79 defined channels with 1 MHz channel spacing, i.e. bandwidth). Thus, Bluetooth uses virtually the entire 2400-2483.5 MHz band, which is a disadvantage due to the high potential for interference. The original motivation for a large number of channels was to spread transmission energy over as many channels as possible, thus maximizing the energy spread. Such a scheme is advantageous in and was originally developed for military systems in which it is desirable to prevent eavesdropping and frequency pattern detection by the enemy.
Consider the example prior art system 40 of FIG. 2. System A comprises a master device 42 that transmits to receiving slave device 44. Similarly, system B comprises a master device 46 that transmits to receiving slave device 48. Different systems (i.e. A and B) coexisting in the same environment are likely to interfere with each other with a probability that increases the more systems there are. Thus, two systems located in an environment that is very dense with frequency hopping systems, are likely to be at different frequencies at most times. There is a probability, however, that at some instance they will be at the same frequency and potentially interfere with each other such as shown in FIG. 3.
For example, the packet stream 30 transmitted by system A eventually interferes with the packet stream 32 transmitted by system B. The collision eventually occurs due to the drift of the crystal oscillator in each device. Assuming both clocks are drifting apart in opposite directions, eventually, the trailer of one packet will overlap with the header of another packet, depending on the difference in their clock frequencies. Theoretically, two systems can maintain the same relative timing indefinitely, but in practice, since they are not synchronized and each system uses its own crystal oscillator, their relative timing will drift over time.
Note that coexisting systems would never interfere or collide with each other if each used an entirely different set of frequencies. At some point, however, the available frequencies will be exhausted and collisions will inevitably occur. Bluetooth, in its original definition, required the use of all 79 channels, 2402 MHz to 2480 MHz with 1 MHz spacing. It was required to utilize all channels in some pseudo-random order. Thus, coexisting systems had to overlap each other at some points in time, potentially resulting in interference.
Historically, FCC regulations mandated the use of at least 75 hopping frequencies in the 2.4 GHz ISM band, which forced this choice (not shared in other regions such as France and Japan at the time, where the hop-sequence was defined with only 23 channels). The power spectral density, however, is instantaneously very high in a frequency hopping spread spectrum transmission. It is only over time, that the energy is spread out over the band in a frequency hopping system, and therefore in many coexistence scenarios, the use of a greater number of hopping frequencies is problematic rather than being advantageous.
Consider a victim system such as the WLAN system 20 that transmits very long packets and uses a large amount of bandwidth compared to the Bluetooth system. The drawback in this coexistence scenario is if a WLAN packet experiences interference, it typically must be retransmitted. If only a portion of the packet is corrupted, the number of retransmissions will typically depend on the particular protocol employed, interleaving, coding, etc. Whether the packet is interfered with for a short time may not reduce the need for retransmission. Thus, Bluetooth transmissions and other frequency hopping schemes like it, can potentially create noticeable interference for other wireless systems because of the large amount of energy instantaneously concentrated in a narrow bandwidth over the entire band of frequencies, thus often making coexistence with other systems problematic.
One solution is to segment long packets into many smaller ones. The disadvantage with this is the increased transmission overhead required for packet headers, trailers, etc. in addition to the processing resources requirement.
Conventional frequency hopping techniques encourage the maximization of the number of hopping channels used as well as the randomization in the hopping sequence, since frequency hopping is typically associated with secure communications (i.e. the avoidance of intentional jamming and eavesdropping). Other than in military applications, however, these properties are not necessarily required. In non-military applications, the goal is simply to transport information from one location to another, possibly in the presence of coexisting systems and multipath fading. Prior art solutions in Bluetooth make use of the original 79-channel complex hopping sequence. These prior art schemes (1) fail to coexist better when a neighboring system is not detected (which is a scenario of high likelihood), (2) do not accommodate for frequency reuse among many coexisting piconets, based on time-axis avoidance, and (3) are not capable of determining which channels are shared between coexisting Bluetooth piconets, since collisions at a specific frequency will not necessarily be repetitive.